1. Field of the Invention
The present invention relates to an image forming apparatus of the electrophotographic process such as a copying machine, a printer or a facsimile machine, and more particularly to an image forming apparatus having a function of forming images on both faces of a recording material.
2. Related Background Art
FIG. 6 shows a cross-sectional view of a copying machine which is capable of forming images on both faces of a recording material.
The image forming apparatus shown in FIG. 6 is designed so that a surface of an image bearing member (photosensitive drum) 1 that rotates in a direction indicated by an arrow R1 is uniformly charged by a charger 2, and an electrostatic latent image is formed by the exposure of exposure means 3. Toner is attached onto the electrostatic latent image and developed as a toner image by a developing device 4. The toner image on the image bearing member 1 is transferred by a transfer charger 5 onto a transfer material P such as paper which is fed and conveyed through a feeding path 12 from a sheet feed cassette 11 by feeding means such as a sheet pickup roller and a conveying roller. The toner (untransferred toner) remaining on the surface of the image bearing member 1 from which the toner image has been transferred, which has not been transferred onto the transfer material P is removed by a cleaning blade (cleaning device) 6. On the other hand, the transfer material P onto which the toner image has been transferred is conveyed to a fixing device (heating portion) 7 by a conveying belt 13, and heated and pressurized by a fixing roller 7a and a pressure roller 7b so that the toner image is fixed onto the surface of the transfer material P. In the case where the toner image is formed on only one face (first face) of the transfer material P, a flapper 14 is disposed at a position shown in FIG. 6 whereby the transfer material P onto which the image has been formed is discharged to the external by a sheet discharge roller 15, thus completing the image formation.
On the other hand, in the case where the toner images are formed on both faces (first face and second face) of the transfer material P, after the toner image has been fixed on the first face as described above, the sheet discharge roller 15 is stopped immediately after a tailing end of the transfer material P passed through the flapper 14, and the flapper 14 is changed over to reversely rotate the sheet discharge roller 15. As a result, the transfer material P is conveyed to a re-conveying belt 31 disposed in a re-feeding path 30 while the transfer material P is guided by a lower face of the flapper 14, and then contained into an intermediate tray 32. After the formation of images on the first faces of the transfer materials P of a given number has been completed and the transfer materials P of the given number have been contained in the intermediate tray 32, the formation of images on the second faces of the transfer materials P starts. The transfer material P within the intermediate tray 32 is fed by a re-feed belt 33 and again supplied to the image bearing member 1 through the feeding path 12. With this operation, the toner image formed on the photosensitive drum 1 is transferred onto the second face of the transfer material P. The transfer material P onto the second face of which the toner image has been transferred is discharged to the exterior of an image forming apparatus body by the sheet discharge roller 15 after the toner image has been fixed onto the second face of the transfer material P in the same manner as in the above-described case where the toner image is formed on only the first face of the transfer material P. As a result, the formation of images on the first and second faces of the transfer material P has been completed.
In the above-described image forming apparatus, a sheet heater 22 is embedded in the image bearing member 1 as shown in FIG. 6 and heats the surface of the image bearing member 1 to a temperature of about 40 to 50xc2x0 C. which is slightly higher than the room temperature under the control. The purposes of heating the surface of the image bearing member 1 are as follows:
(1) Prevention of Smeared Image:
If a large number of one-face copies are continuously conducted under the circumstances of high temperature and high humidity, because the temperature of the image bearing member is lowered, water molecules are adsorbed on the surface of the image bearing member to lower the surface resistance, the electrostatic latent image is disordered. In order to prevent the above drawback, the warmth of the image bearing member is retained so that the water molecules can be appropriately heated and evaporated.
(2) Stabilization of Photosensitive Characteristics:
In the case where the image bearing member is formed of a photosensitive member, the photosensitive property such as a charging capability or a dark decay depends on temperature, and it is desirable to hold the surface temperature constant in order to hold those characteristics constant.
(3) Stabilization of Cleaning Characteristics:
Because the cleaning blade is generally made of urethane rubber, those visco-elastic characteristic depends on temperature, and therefore it is desirable to hold the temperature constant.
In order to achieve the above-described purposes, the surface temperature of the image bearing member 1 is detected by a thermistor 21, and the on/off operation of power supply to the sheet heater 22 is controlled on the basis of the detected result by control means 23 so that the surface temperature of the image bearing member 1 is controlled to the above-described temperature of 40 to 50xc2x0 C.
Incidentally, a method of forming the images on both faces of the transfer material by using the intermediate tray 32 as shown in FIG. 6 is effective in an analog copying machine in which a light is irradiated onto an original and its reflected light is directly irradiated onto a photosensitive member to form an image on the photosensitive member. The reason is stated below. That is, in case of the analog copying machine, the replacing work of the original (or reverse work) is required to copy different images on both faces of the recording material (transfer material). In order to reduce a copying wait time during the replacing work, a method in which one-face copying is conducted on the recording materials as many as possible without replacing the original by another original, the original is thereafter replaced by another original and an image on another original is copied on a back surface of the recording material on a front surface of which an image has been copied is desirable to increase the copying speed. However, if no intermediate tray is provided, a large number of recording materials one surface of which has been copied cannot be stored. Therefore, it is necessary to again return to the first original sooner.
On the other hand, in case of the digital copying machine adapted to form an image on a photosensitive member in response to an image signal, if the image data of every plural originals is saved, the first-original image and the second-original image can be formed on the photosensitive member at random without requiring the wait time for the original replacement as in the analog copying machine. As a result, a large number of both-face copies can be rapidly conducted even if no intermediate tray is provided. Accordingly, the recording material one face of which has been copied and which stops once on the intermediate tray in the analog copying machine can be again conveyed at the transfer position of the photosensitive member without stopping halfway in the digital copying machine.
When only one face of the transfer material P is thus copied by the copying machine using no intermediate tray, because the transfer material P absorbs more than a little heat from the image bearing member 1, the temperature of the image bearing member is lowered. On the contrary, in the image forming process for forming images on both faces of the transfer material, the image forming process for the second face is again sequentially conducted (after a short period of time) after the transfer material P has passed through the fixing device 7 at the time of the image formation for the first face. For that reason, the heat given to the transfer material P from the fixing device 7 is given to the image bearing member 1 during the transfer process with the result that the image bearing member 1 rises in temperature. In this case, the toner attached on an edge face of the cleaning blade 6 which is in contact with the image bearing member 1 is agglomerated and consolidated by the influence of the pressure and heat, resulting in a deterioration of the cleaning performance.
The present invention has been made in view of the above-described problems, and therefore an object of the present invention is to provide an image forming apparatus which is capable of performing both of one-face copying and both-face copying with a high quality.
Another object of the present invention is to provide an image forming apparatus which suppresses the rising of temperature of an image bearing member when conducting both-face copying.
Still another object of the present invention is to provide an image forming apparatus, including: an image bearing member; a heater for heating the image bearing member; and control means for controlling the heater so that a temperature of the image bearing member is maintained at a set temperature, wherein the set temperature when images are formed on both faces of a recording material is set to be lower than the set temperature when an image is formed on one face of a recording material.
Yet still another object of the present invention is to provide an image forming apparatus, including: an image bearing member; a heater for heating the image bearing member; and control means for controlling the heater so that a temperature of the image bearing member is maintained at a set temperature, wherein the control means controls the temperature of the image bearing member when an image is formed on one face of a recording material, and continues to turn off a power supply to the heater when an image is formed on both faces of a recording material.
Other objects of the present invention will become apparent by reading the following detailed description with reference to the accompanying drawings.